Since the discovery in 1911 that mercury is electrically superconductive when cooled to 4° K, many materials have been shown to become superconductive below some critical temperature or transition temperature Tc.
Superconductors are divided into type 1 and type 2. Type 1 superconductors tend to have a low Tc and the transition between the non-superconducting and superconducting state typically occurs over a temperature span of less than 1° K. Type 1 superconductors are mainly pure metals that conduct electricity at room temperature. All type 1 superconductors only become superconducting at temperatures within less than 20 degrees K of absolute zero. Examples are tungsten (Tc=0.15° K), titanium (Tc=0.40° K), aluminum (Tc=1.175° K), tin (Tc=3.72° K) and lead (Tc=7.2° K). All these values are at normal pressures. The type 1 material with the highest known Tc is sulfur, for which Tc is about 17° K; however, sulfur needs to be compressed to 930,000 atmospheres to become superconducting and 1.6 million atmospheres to achieve a Tc of 17° K.
In type 2 superconductors, the transition between states usually extends over a broader range of temperature, typically 5° K. While type 1 and type 2 Tc values overlap, the majority of type 2 materials superconduct at much higher temperatures than any type 1 material and include the so-called “high-Tc” superconductors, which are typically ceramic materials. Examples of type 2 superconductors are the elements vanadium (Tc=5.4° K), technetium (Tc=7.8° K) and niobium (Tc=9.25° K). These and all further values of Tc will be understood to be at atmospheric pressure.
The first superconductive wire was composed of Nb0.6Ti0.4. The first ceramic superconductor discovered, in 1986, was La0.85Ba0.15CuO4, with a Tc of 35° K). A common high-Tc material is the compound YBa2Cu3O7, often referred to as “YBCO” or “123”. YBCO has a Tc of 93° K. The highest Tc currently known is 138° K for the compound Hg0.8Tl0.2Ba2Ca2Cu3O8.33. Practical applications have been sought for superconductive materials. One use is in power transmission, in which the absence of electrical resistance would significantly reduce power losses. Power transmission over long distances is precluded by the requirement to maintain low temperatures by some means such as using liquid nitrogen. There have been specialized commercial applications; for example, power transmission over relatively short distances through limited spaces such as tunnels is greatly increased by using superconductors, to a degree not achievable by using conventional power cables.
Other behaviors are associated with superconductivity, in particular the Meissner effect, whereby an article in a superconductive state can strongly deflect a magnetic field; the superconductor and a magnet will produce a mutually repulsive force. This effect is used in magnetic levitation (maglev) transportation systems that are in development.